Colon cancer is a major health challenge, ranking as the second leading cause of cancer deaths. Most cases are initiated by mutations in the tumor suppressor Adenomatous polyposis coli (APC). APC is best known as a key negative regulator of the Wnt cell-cell signaling pathway, which shapes cell fates in virtually every tissue and organ, regulates homeostasis of stem cells in several tissues, and is inappropriately activated in many cancers. APC also plays Wnt-independent roles in regulating the cytoskeleton, thus facilitating processes like high-fidelity chromosome segregation, which also are disrupted in cancer. Our lab's long term goal is to determine how APC and its protein partners regulate both Wnt signaling and the cytoskeleton during normal development and homeostasis, and how that process goes wrong in colon and other cancers. Our lab played important roles in defining how APC acts in the destruction complex, which targets the Wnt effectors sscatenin for ubiquitination and destruction, and also identified APC as a regulator of cytoskeletal events. To do so, we established powerful dual model systems to study this in parallel in intact animals during Drosophila development and in cultured human cells, using a highly multidisciplinary approach combining state-of-the-art genetic, cell biological, and biochemical tools. Understanding mechanisms by which APC and its protein partners regulate both Wnt signaling and the cytoskeleton during normal development will thus advance both basic science and clinical applications. While we now possess textbook models for both Wnt regulation and APCs cytoskeletal function, it is becoming increasingly clear that these paradigms are oversimplified-in fact APC's mechanistic roles in the destruction complex and in cytoskeletal regulation remain largely mysterious. Our work in the past funding period challenged existing hypotheses for APC function in Wnt regulation, including our own, demonstrating that APC does not play essential nuclear roles in Wnt regulation nor does it play an essential role in localizing the destruction complex. Instead, work from our lab and others suggests the innovative hypothesis underlying Aim 1, in which APC regulates a dynamic catalytic cycle of the destruction complex, which is essential for sscatenin transfer to the E3 ubiquitin ligase. In Aim 1 we will define mechanisms by which APC and Axin act in a dynamic destruction complex, identify novel protein partners that regulate the cycle of conformational change, and determine how Wnt signaling alters destruction complex dynamics to turn it off. Our data also suggest textbook views of APC's roles as a cytoskeletal regulator must be significantly modified. We hypothesize APC is not an essential regulator of mitosis, but instead acts to ensure high fidelity chromosome segregation, working as part of a multiprotein complex, and with its action buffered by mitotic checkpoints that can partially compensate for its loss-this underlies Aim 2. In this Aim we will determine how APC and its partners act to regulate centrosome migration and thus ensure high fidelity chromosome segregation, and explore how Chk2 and other checkpoint regulators buffer the errors that occur in its absence.